Systems and methods for selectively decoupling a parallel extended instruction pipeline

ABSTRACT

Systems and methods for selectively decoupling a parallel extended processor pipeline. A main processor pipeline and parallel extended pipeline are coupled via an instruction queue. The main pipeline can instruct the parallel pipeline to execute instructions directly or to begin fetching and executing its own instructions autonomously. During autonomous operation of the parallel pipeline, instructions from the main pipeline accumulate in the instruction queue. The parallel pipeline can return to main pipeline controlled execution through a single instruction. A light weight mechanism in the form of a condition code as seen by the main processor is designed to allow intelligent decision maximizing overall performance to be made in run-time if further instructions should be issued to the parallel extended pipeline based on the queue status.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 60/721,108 titled “SIMD Architecture and Associated Systems and Methods,” filed Sep. 28, 2005, the disclosure of which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to embedded microprocessor architecture and more specifically to systems and methods for selectively decoupling an extended instruction pipeline from a main pipeline in an microprocessor-based system.

BACKGROUND OF THE INVENTION

Processor extension logic is utilized to extend a microprocessor's capability. Typically, this logic is in parallel to and accessible by the main processor pipeline. It is often used to perform specific, repetitive, computationally intensive functions thereby freeing up the main processor pipeline.

In conventional microprocessors, there are essentially two types of parallel pipeline architectures: tightly coupled and loosely coupled, or decoupled. In the former, instructions are fetched and executed serially in the main processor pipeline. If the instruction is an instruction to be processed by the extension logic, the instruction is sent to that logic. Because every instruction originates from the main pipeline the two pipelines are said to be tightly coupled. This limits the degree of concurrency exploitable between the pipelines.

In the second architecture, the parallel instruction pipeline containing the extension logic is capable of fetching and executing its own instructions and hence maximizing concurrency. However, control and synchronization between the two pipelines becomes difficult when programming a processor having such a decoupled architecture. Thus, there exists a need for a parallel pipeline architecture that can fully exploit the advantages of parallelism without suffering from the design complexity of loosely or completely decoupled pipelines.

SUMMARY OF THE INVENTION

Accordingly, at least one embodiment of the invention provides a microprocessor architecture. The microprocessor architecture according to this embodiment comprises a first processor instruction pipeline, comprising a front end portion and a rear portion, a second processor instruction pipeline, comprising a front end portion and a rear portion, and an instruction queue coupling the first and second instruction pipeline between their respective front end and rear portions.

Another embodiment of the invention provides a method of dynamically decoupling a parallel extended processor pipeline from a main processor pipeline. The method according to this embodiment comprises sending an instruction from the main processor pipeline to the parallel extended processor pipeline instructing the parallel extended processor pipeline to operate autonomously, operating the parallel extended processor pipeline autonomously, storing subsequent instructions from the main processor pipeline to the parallel extended processor pipeline in an instruction queue, executing an instruction with the parallel extended processor pipeline to cease autonomous execution, and thereafter executing instructions supplied by the main processor pipeline in the queue.

Still a further embodiment of the invention provides a method of performing dynamically controlled parallel instruction processing in a microprocessor. The method according to this embodiment comprises fetching and executing instructions with a main processor pipeline, sending instructions from the main processor pipeline to a parallel extended processor pipeline via an instruction queue coupling the two pipelines, and if the instruction is to an instruction to be executed by the parallel extended pipeline, executing that instruction with the parallel extended pipeline, otherwise if the instruction is an instruction instructing that parallel extended pipeline to begin autonomous execution, thereafter fetching and executing instructions autonomously with the parallel extended pipeline independent of the main pipeline's instruction fetches, and storing instructions from main pipeline for the parallel extended pipeline in the instruction queue until autonomous processing has ceased.

These and other embodiments and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the present disclosure, but are intended to be exemplary only.

FIG. 1 is a functional block diagram illustrating a microprocessor-based system including a main processor core and a SIMD media accelerator according to at least one embodiment of the invention;

FIG. 2 is a block diagram illustrating a conventional multistage microprocessor pipeline having a pair of parallel data paths;

FIG. 3 is a block diagram illustrating another conventional multiprocessor design having a pair of parallel processor pipelines;

FIG. 4 is a block diagram illustrating a dynamically decoupleable multi-stage microprocessor pipeline according to at least one embodiment of the invention; and

FIG. 5 is a flow chart detailing the steps of a method for sending instructions for operating a main processor pipeline and an extended processor pipeline according to at least one embodiment of the invention; and

FIG. 6 is a flow chart detailing the steps of a method for dynamically decoupling an extended processor pipeline from a main pipeline according to at least one embodiment of the invention.

DETAILED DESCRIPTION

The following description is intended to convey a thorough understanding of the embodiments described by providing a number of specific embodiments and details involving microprocessor architecture and systems and methods for selectively decoupling an extended instruction pipeline from a main instruction pipeline. It should be appreciated, however, that the present invention is not limited to these specific embodiments and details, which are exemplary only. It is further understood that one possessing ordinary skill in the art, in light of known systems and methods, would appreciate the use of the invention for its intended purposes and benefits in any number of alternative embodiments, depending upon specific design and other needs.

Referring now to FIG. 1, a functional block diagram illustrating a microprocessor-based system 5 including a main processor core 10 and a SIMD media accelerator 50 according to at least one embodiment of the invention The diagram illustrates a microprocessor 5 comprising a standard single instruction single data (SISD) processor core 10 having a multistage instruction pipeline 12 and a SIMD media engine 50. In various embodiments, the processor core 10 may be a processor core such as the ARC 700 embedded processor core available from ARC, International of Elstree, United Kingdom, and as described in provisional patent application No. 60/572,238 filed May 19, 2004 entitled “Microprocessor Architecture” which, is hereby incorporated by reference in its entirety. Alternatively, in various embodiments, the processor core may be a different processor core.

In various embodiments, a single instruction issued by the processor pipeline 12 may cause up to 16 16-bit elements to be operated on in parallel through the use of the 128-bit data path 55 in the media engine 50. In various embodiments, the SIMD engine 50 utilizes closely coupled memory units. In various embodiments, the SIMD data memory 52 (SDM) is a 128-bit wide data memory that provides low latency access to and from the 128-bit vector register file 51. The SDM contents are transferable to and from system main memory via a DMA unit 54 thereby freeing up the processor core 10 and the SIMD core 50. In various embodiments, a SIMD code memory 56 (SCM) allows the SIMD unit to fetch instructions from a localized code memory, allowing the SIMD pipeline to dynamically decouple from the processor core 10 resulting in truly parallel operation between the processor core and SIMD media engine as will be discussed in greater detail in the context of FIGS. 4-6.

Therefore, in various embodiments, the microprocessor architecture will permit the processor-based system 5 to operate in both closely coupled and decoupled modes of operation. In the closely coupled mode of operation, the SIMD program code fetch is exclusively handled by the main processor core 10. In the decoupled mode of operation, the SIMD pipeline 50 executes code fetched from a local memory 56 independent of the processor core 10. The processor core 10 may therefore instruct the SIMD pipeline 50 to execute autonomously in this de-coupled mode, for example, to perform video tasks such as audio processing, entropy encoding/decoding, discrete cosine transforms (DCTs) and inverse DCTs, motion compensation and de-block filtering.

Referring now to FIG. 2, a block diagram illustrating a conventional multistage microprocessor pipeline having a pair of parallel data paths is depicted. In a microprocessor employing a variable-length pipeline, data paths required to support different instructions typically have a different number of stages. Data paths supporting specialized extension instructions for performing digital signal processing or other complex but repetitive functions may be used only some of the time during processor execution and remain idle otherwise. Thus, whether or not these instructions are currently needed will effect the number of effective stages in the processor pipeline.

Extending a general-purpose microprocessor with application specific extension instructions can often add significant length to the instruction pipeline. In the pipeline of FIG. 2, pipeline stages F1 to F4 at the front end 100 of the processor pipeline are responsible for functions such as instruction fetch, decode and issue. These pipeline stages are used to handle all instructions issued by the microprocessor. After these stages, the pipeline splits into parallel data paths 110 and 115 incorporating stages E1-E3 and D1-D4 respectively. These parallel sub-paths represent pipeline stages used to support different instructions/data operations. For example, stages E1-E3 may be the primary/default processor pipeline, while stages D1-D4 comprise the extended pipeline designed for processing specific instructions. This type of architecture can be characterized as coupled or tightly coupled to the extent that regardless of whether instructions are destined for default pipeline stages E1-E3 or extended pipeline D1-D4, they all must pass through stages F1-F4, until a decision is made as to which portion of the pipeline will perform the remaining processing steps.

By using the single pipeline front-end to fetch and issue all instructions, the processor pipeline of FIG. 2 achieves the advantage that instructions can be freely intermixed, irrespectively of whether the instructions are executed by the data path in sub-paths E1-E3 or D1-D4. Thus, all instructions appear as a single thread of program execution. This type of pipeline architecture also has the advantage of greatly simplified program design and debugging, thereby reducing the time to market in product developments. It is admittedly a highly flexible architecture. However, a limitation of this architecture is that the sequential nature of instruction execution significantly limits the exploitable parallelism between the data paths that could otherwise be used to improve overall performance. This negatively effects performance relative to other parallel pipeline architectures.

FIG. 3 is a block diagram illustrating another conventional multiprocessor architecture having a pair of parallel instruction pipelines. The processor pipeline of FIG. 3 contains a front end 120 comprised of stages F1-F4 and a rear portion 125 comprised of stages E1-E3. However, the processor also contains a parallel data path having a front end 135 comprised of front end stages G1-G2 and rear portion 140 comprised of stages D1-D4. Unlike the architecture of FIG. 2, this architecture contains truly parallel pipelines to the extent that both front portions 420 and 435 each can fetch instructions separately. This type of parallel architecture may be characterized as loosely coupled or decoupled because the application specific extension data path G1-G2 and D1-D4 is autonomous and can execute instructions in parallel to the main pipeline consisting of F1-F4 and E1-E3. This arrangement enhances exploitable parallelism over the architecture depicted in FIG. 2. However, as the two parallel pipelines become independent, mechanisms are required to synchronize their operations, as represented by dashed line 130. These mechanisms, typically implemented using specific instructions and bus structures which, are often not a natural part of a program and are inserted as after-thoughts to “fix” the disconnect between main pipeline and extended pipeline. As consequence of this, the resulting program utilizing both instruction pipelines becomes difficult to design and optimize.

Referring now to FIG. 4, a block diagram illustrating a dynamically decoupleable multi-stage microprocessor pipeline according to at least one embodiment of the invention is provided. The pipeline architecture according to this embodiment ameliorates at least some and preferably most or all of the above-noted limitations of conventional parallel pipeline architectures. This exemplary pipeline depicted in FIG. 4 consists of a front end portion 145 comprising stages F1-F4, a rear portion 150 comprising stages E1-E3, and a parallel extendible pipeline having a front portion 160 comprising stages G1-G2 and a rear portion 165 comprising stages D1-D4. In the pipeline depicted in FIG. 4, instructions can be issued from the CPU to the extendible pipeline D1 to D4. To decouple the extendible pipeline D1 to D4 from the front portion 145 of the main pipeline F1 to F4, a queue 155 is added between the two pipelines. The queue serves to delay execution of instructions issued by the front end portion 145 of the main pipeline if the extension pipeline is not ready. A tradeoff can be made during system design to decide on how many entries should be in the queue 155 to insure that the extension pipeline is sufficiently decoupled from the main pipeline. Additionally, in various embodiments, the main pipeline can issue a Sequence Run (vrun) instruction to instruct the extension pipeline to use its own front end 160, G1 to G2 in the diagram, to execute instruction sequences stored in a record memory 156, causing the extension pipeline to fetch and execute instructions autonomously. In various embodiments, while the extension pipeline, G1-G2 and D1-D4, is performing operations, the main pipeline can keep issuing extension instructions that accumulate in the queue 155 until the extension pipeline executes a Sequence Record End (vendrec) instruction. After the vendrec instruction is issued, the extension resumes executing instructions issued to the queue 155.

Therefore, instead of trying to get what effectively becomes two independent processors to work together as in the pipeline depicted in FIG. 3, the pipeline depicted in FIG. 4 is designed to switch between being coupled, that is, executing instructions for the main pipeline front end 145, and being decoupled, that is, during autonomous runtime of the extended pipeline. As such, the instructions vrun and vendrec, which dynamically switch the pipeline between the coupling states, can be designed to be light weight, executing in, for example, a single cycle. These instructions can then be seen as parallel analogs of the conventional call and return instructions. That is, when instructing the extension pipeline to fetch and execute instructions autonomously, the main processor pipeline is issuing a parallel function call that runs concurrently with its own thread of instruction execution to maximize speedup of the application. The two threads of instruction execution eventually join back into one after the extension pipeline executes the vendrec instruction which is the last instruction of the program thread autonomously executed by the extension pipeline.

In addition to efficient operation, another advantage of this architecture is that during debugging, such as, for example, instruction stepping, the two parallel threads can be forced to be serialized such that the CPU front portion 145 will not issue any instruction after issuing vrun to the extension pipeline until the latter fetches and executes the vendrec instruction. In various embodiments, this will give the programmer the view of a single program thread that has the same functional behavior of the parallel program when executed normally and hence will greatly simplify the task of debugging.

Another advantage of the processor pipeline containing a parallel extendible pipeline that can be dynamically coupled and decoupled is the ability to use two separate clock domains. In low power applications, it is often necessary to run specific parts of the integrated circuit at varying clock frequencies, in order to reduce and/or minimize power consumption. Using dynamic decoupling, the front end portion 145 of the main pipeline can utilize an operating clock frequency different from that of the parallel pipeline 165 of stages D1-D4 with the primary clock partitioning occurring naturally at the queue 155 labeled as Q in the FIG. 4.

Referring now to FIG. 5, a flow chart of an exemplary method for sending instructions from a main processor pipeline to an extended processor pipeline according to at least one embodiment of the invention is depicted. Operation of the method begins in step 200 and proceeds to step 205, where an instruction is fetched by the main processor pipeline. In step 210, because the instruction is determined to be one for processing by the parallel extended pipeline, the instruction is passed from the main pipeline to the parallel extended pipeline via an instruction queue coupling the two pipelines. In various embodiments, if the parallel extended pipeline is currently processing instructions from the queue, that instruction will be processed in turn by the parallel extended pipeline as specified in step 220. Otherwise, the instruction will remain in the queue until the parallel extended pipeline has ceased its autonomous operation. In step 225, while the instruction is either sitting in the queue or being processed by the parallel pipeline, the main pipeline is able to continue processing instructions. The queue provides a mechanism for the main pipeline to offload instructions to the parallel extended pipeline without stalling the main pipeline. Operation of the method stops in step 230.

Referring now to FIG. 6, this Figure is a flow chart of an exemplary method for dynamically decoupling an extended processor pipeline from a main pipeline according to at least one embodiment of the invention. Operation of the method begins in step 300 and proceeds to step 305 where the main processor pipeline sends a run instruction to the parallel extended pipeline via the instruction queue coupling the pipelines. In step 310, the parallel pipeline retrieves the run instruction from the queue. As noted above, this may occur instantly or after the parallel pipeline has retrieved and processed other instructions in front of the run instruction in the queue. In various embodiments, this run instruction will specify a location in a record memory accessible by the parallel extended pipeline of a starting location of a sequence of recorded instructions. Next, in step 315, based on receipt of the run instruction, the parallel extended pipeline begins executing the series of recorded instructions, that is, it begins autonomous operation. In various embodiments this comprises fetching and executing its own instructions independent of the main pipeline's instruction stack. Also, in various embodiments, the parallel extended pipeline may operate at another clock frequency that the main pipeline, such as, for example, a fractional percentage (i.e., ½, ¼, etc.). Concurrent to the parallel extended pipeline's autonomous execution, the main processor pipeline can continue sending instructions to the parallel extended pipeline as depicted in step 320. Then, in step 325, after the parallel pipeline has processed an end instruction recorded at the end of the sequence of recorded instructions, autonomous operation of that pipeline ceases. In step 330, the parallel pipeline returns to the queue to process any queued instructions received from the main pipeline. In step 335, the parallel extended pipeline continues processing instructions issued by the main pipeline that appear in the queue until an instruction to begin autonomous operation is received.

As discussed above, in the microprocessor architecture according to the various embodiments of the invention, a main processor pipeline is extended through a dynamically coupled parallel SIMD instruction pipeline. In various embodiments, the main processor pipeline may issue instructions to the extended pipeline through an instruction queue that effectively decouples the extended pipeline. In various embodiments, the extended SIMD pipeline is also able to run prerecorded macros that are stored in a local SIMD instruction memory so that a single macro instruction sent to the SIMD pipeline via the queue allows many pre-determined instructions to be executed as discussed in commonly assigned U.S. patent applications XX/XXX,XXX titled, “Systems and Methods for Recording Instructions Sequences in a Microprocessor Having a Dynamically Decoupleable Extended Instruction Pipeline,” filed concurrently herewith, the disclosure of which is hereby incorporated by reference in its entirety. This architecture, among other things, allows the SIMD media engine (the extended pipeline) to operate in parallel with the primary pipeline (processor core) and allows the processor core to operate far in advance of the parallel SIMD pipeline.

One consideration of using an instruction queue to decouple the extended SIMD pipeline from the processor core (main pipeline) is that it becomes possible for the processor core to issue too many instructions causing the queue to become full. When the main processor pipeline can no longer issue instructions to the queue, the pipeline will have to stall until the queue frees up a slot for the instruction that caused the pipeline to stall. Pipeline stalls have a negative effect on overall system performance. In this case in particular, a pipeline stall means that the processor core will stop being able to operate in parallel, therefore negating the gains derived from the dynamically decoupled extended parallel SIMD pipeline.

Therefore, in order to prevent the main processor pipeline from issuing instructions to the queue when it is full, thereby causing the main pipeline to stall, in various embodiments, the SIMD pipeline queue uses condition codes to notify the processor pipeline of the condition of the queue. In various embodiments, the SIMD queue sets a condition code of QF for queue nearly full whenever there are less than a predetermined number of empty slots remaining in the queue. In various embodiments, this number may be 16. However, in various embodiments, the number may be different than 16. In various embodiments, the SIMD queue sets a condition code of QNF as the opposite of QF when more than the predetermined number of slots remain available.

In various embodiments, rather than using several instructions to load these status values and test the value before branching on the test result, two conditional branch instructions using these condition codes directly test for such conditions, thereby reducing the number of instructions required to perform this task. In various embodiments, these instructions will only branch when the condition code used is set. In various embodiments, these instructions may have the mnemonic “BQF” for branch when queue is nearly full and “BQNF” for branch when queue is not nearly full. Such condition codes make the queue full status an integral part of the main processor programming model and make it possible to make frequent light-weight intelligent decisions by software to maximize overall performance. These condition codes are maintained by the queue itself based on the queue's status. The instruction to check the condition code are branch instructions that are specified to check the particular condition codes. In various embodiments of the invention, checking of the condition code is done by placing condition code checking branch instructions where necessary, such as before issuing any instructions to the extended pipeline. Thus, the condition codes provide an easy mechanism for preventing main pipeline stalls caused by trying to issue instructions to a full queue.

These two conditional branch instructions allow the main processor pipeline to regularly check the status of the queue before issuing more instructions into the extended SIMD pipeline queue. The main processor core can use these instructions to avoid stalling the processor when the queue is full or nearly full, and branch to another task that does not involve the SIMD engine until these queue conditions change. Therefore, these instructions provide the processor with an effective and relatively low overhead means of scheduling work load on the available resources while preventing main pipeline stalls.

The embodiments of the present inventions are not to be limited in scope by the specific embodiments described herein. For example, although many of the embodiments disclosed herein have been described with reference to systems and dynamically decoupling a parallel pipeline in a microprocessor-based system having a main instruction pipeline and an extended instruction pipeline, the principles herein are equally applicable to other aspects of microprocessor design and function. Indeed, various modifications of the embodiments of the present inventions, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such modifications are intended to fall within the scope of the following appended claims. Further, although some of the embodiments of the present invention have been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the embodiments of the present inventions can be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breath and spirit of the embodiments of the present inventions as disclosed herein. 

1. A microprocessor architecture comprising: a first processor instruction pipeline, comprising a front end portion and a rear portion; a second processor instruction pipeline, comprising a front end portion and a rear portion; and an instruction queue coupling the first and second instruction pipeline between their respective front end and rear portions.
 2. The microprocessor architecture according to claim 1, where the instruction queue is located in the second instruction pipeline between that pipeline's front end and rear portions.
 3. The microprocessor architecture according to claim 1, wherein the queue is configured to store instructions issued by the first instruction pipeline to the second instruction pipeline.
 4. The microprocessor architecture according to claim 1, wherein the first instruction pipeline is configured to be able to instruct the second instruction pipeline to operate autonomously.
 5. The microprocessor architecture according to claim 4, wherein operating autonomously comprises fetching and executing its own instructions via the second pipeline's front end portion.
 6. The microprocessor architecture according to claim 4, wherein operating autonomously comprises operating on a different clock frequency than the first instruction pipeline.
 7. The microprocessor architecture according to claim 5, wherein instructions issued to the second instruction pipeline accumulate in the queue during its autonomous operation.
 8. The microprocessor architecture according to claim 7, wherein the instruction queue comprises at least one condition code.
 9. The microprocessor architecture according to claim 8, wherein the at least one condition code comprises a code indicative of at least one state of the queue selected from the group consisting queue having less than a predetermined number of free slots, queue having more than a predetermined of free slots, and queue full.
 10. The microprocessor architecture according to claim 9, wherein the first processor instruction pipeline uses the at least one condition code to determine whether to send an instruction to the queue or to branch to another instruction that does not require the second instruction pipeline.
 11. The microprocessor architecture according to claim 7, wherein the second instruction pipeline is adapted to return from autonomous operation to first instruction pipeline controlled operation by executing a return instruction.
 12. The microprocessor architecture according to claim 11, wherein instructions accumulated in the queue are executed by the second instruction pipeline when it returns from autonomous operation.
 13. A method of dynamically decoupling a parallel extended processor pipeline from a main processor pipeline comprising: sending an instruction from the main processor pipeline to the parallel extended processor pipeline instructing the parallel extended processor pipeline to operate autonomously; operating the parallel extended processor pipeline autonomously; storing subsequent instructions from the main processor pipeline to the parallel extended processor pipeline in an instruction queue; executing an instruction with the parallel extended processor pipeline to cease autonomous execution; and thereafter executing instructions supplied by the main processor pipeline in the queue.
 14. The method according to claim 13, further comprising executing an instruction on the main processor pipeline to check a condition code of the instruction queue before sending subsequent instructions to the queue.
 15. The method according to claim 14, further comprising either branching to another instruction that doesn't require the parallel extended processor pipeline or sending the instruction to the instruction queue based on the condition code.
 16. The method according to claim 13, wherein operating the parallel extended processor pipeline autonomously comprises fetching and executing its own instructions via that pipeline's own front end, independent of instructions fetched and executed by the main processor pipeline.
 17. The method according to claim 13, wherein operating the parallel extended processor pipeline autonomously comprises operating at a different clock frequency than the main processor pipeline.
 18. The method according to claim 13, wherein the main processor pipeline continues to fetch and execute instructions while the parallel extended processor pipeline is operating autonomously.
 19. The method according to claim 13, wherein, executing an instruction with the parallel extended processor pipeline to cease autonomous execution comprises returning from autonomous operation to first instruction pipeline controlled operation without being instructed to do so by the first instruction pipeline.
 20. A method of performing dynamically controlled parallel instruction processing in a microprocessor comprising: fetching and executing instructions with a main processor pipeline; sending instructions from the main processor pipeline to a parallel extended processor pipeline via an instruction queue coupling the two pipelines; and if the instruction is to an instruction to be executed by the parallel extended pipeline, executing that instruction with the parallel extended pipeline; otherwise if the instruction is an instruction instructing that parallel extended pipeline to begin autonomous execution, thereafter fetching and executing instructions autonomously with the parallel extended pipeline independent of the main pipeline's instruction fetches, and storing instructions from main pipeline for the parallel extended pipeline in the instruction queue until autonomous processing has ceased. 